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I think my thermostat is stuck open. I live in Phoenix and when it cools down the check engine light pops on and reads coolant when I plug it in. When it hot out the light shuts off. Anybody ever change the thermostat? If so should I install OEM, replace gasket, do I need to drain coolant?
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I havent replaced one on the hummer, but its pretty straight forward. Just go buy a new one and ask the guy at the counter if you need anything else. No need to drain the coolant. You might spill a little on the ground but it wont be much. Pull upper radiator hose (should be on the water pump) and pull out the thermostat, put new one in and replace hose.
Really easy, shouldn't need anything aside from the thermo, but ask anyways I too haven't done an H3, and be sure to do it when the rad and engine are cold.
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Also, Im not sure why the light comes on. Could be a low coolant problem, or something fancy like its beginning to take on a charge. In that case you would have to change it. If you have a volt meter, pull the radiator cap off, put your negative lead (of the volt meter) on the negative of the battery, then put the positive lead in the coolant (careful not to touch the radiator, just the fluid). If it shows a charge greater than .25 volts it needs to be changed. Let us know what you fing out.
Over time the additives in the coolant deplete. Any time you have fluid moving through dissimilar metals there is a chance it can obtain a charge. The term is called electrolysis. The electrolysis can actually pit the metal in the coolant system. There is not a set "standard" that I know of, some say anything over .20 volts is bad, others say higher. Google coolant system electrolysis, and maybe something will pop up. I was only trying to give the original poster another avenue to look at, while he has attempting to find the cause of his DIC light. Sometimes you just have to throw the book out and learn from the guys with the dirty hands.